Problem: Omar is 18 years older than Christopher. Three years ago, Omar was 4 times as old as Christopher. How old is Christopher now?
Solution: We can use the given information to write down two equations that describe the ages of Omar and Christopher. Let Omar's current age be $o$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $o = c + 18$ Three years ago, Omar was $o - 3$ years old, and Christopher was $c - 3$ years old. The information in the second sentence can be expressed in the following equation: $o - 3 = 4(c - 3)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to use our first equation for $o$ and substitute it into our second equation. Our first equation is: $o = c + 18$ . Substituting this into our second equation, we get the equation: $(c + 18)$ $-$ $3 = 4(c - 3)$ which combines the information about $c$ from both of our original equations. Simplifying both sides of this equation, we get: $c + 15 = 4 c - 12$ Solving for $c$ , we get: $3 c = 27$ $c = 9$.